Watercraft such as high-speed powerboats typically include any of a variety of design features to improve speed, directional stability, and maneuverability. High-speed powerboats typically use dynamic lift, referred to as planing, to reduce resistance created by wave generation and to increase speed. Further, such watercraft often incorporate features to improve directional stability and control during operation.
One class of commonly used watercraft hull is the planing hull (for example, as shown in prior art FIGS. 1A and 1B), which is configured to create positive dynamic pressure so that its draft, or vertical distance to which the hull's keel extends below the waterline, decreases as the speed of the watercraft increases. In other words, a portion of the hull loses contact with the water, or lifts out of the water. The degree by which the front or fore portion of a hull lifts out the water is referred to as the trim angle. Dynamic lift reduces the wetted surface of the hull and, therefore, also reduces drag. However, planing hulls generally suffer from a resistance paradox. At speeds from a standstill to a speed about equal to 1.5 times the waterline length of the watercraft (“transition speed”), the watercraft is in “displacement mode,” meaning that it has not yet reached planing speed and is displacing water as it moves forward. After reaching the transition speed, the watercraft goes into a “transition mode,” in which it is no longer operating in displacement mode or planing mode. Within transition mode, the watercraft has a pronounced bow-up trim. As speed continues to increase, the watercraft moves out of the transition mode and into “planing mode,” in which the trim levels out and the bow of the watercraft lowers somewhat (that is, the trim angle decreases). As the trim angle decreases, the resistance increases linearly with the dynamic pressure, which increases exponentially. This results in extremely high power requirements for very small increases in speed when trim angles are less than about 3 degrees.
Stepped planing hulls were developed to overcome this problem (for example, as shown in prior art FIG. 2). Stepped hull designs incorporate transverse discontinuities, or “steps,” aft of the watercraft's center of gravity and center of pressure. These steps are generally transverse or substantially transverse (perpendicular to the watercraft's centerline), and break the one large, low-aspect-ratio planing hull into multiple high-aspect-ratio planing surfaces, thereby making the hull more efficient. Further, by providing multiple planing surfaces, the trim angle variation with speed is essentially eliminated, breaking the resistance paradox encountered with non-stepped or prismatic planing hulls. A stepped planing hull may be operated with the least drag and the optimal trim angle under all speeds. This makes the resistance increase more linearly with speed, rather than exponentially, and enables the boat to reach much higher speeds, or operate at higher efficiencies than the non-stepped planing hull.
However, the steps of a stepped hull cause a reduction in the wetted area of the hull at high speeds. Although this is favorable for speed and efficiency, it can adversely affect the directional stability and maneuverability of the watercraft. The more the wetted area of the hull is reduced, the more susceptible the watercraft becomes to yawing or uncontrolled turning at high speeds. In order to reintroduce yaw stability, some stepped hull watercraft include features such as strakes and pads, but these have largely been ineffective.
Other stepped hull watercraft include transverse air channels incorporated into the steps, which introduce air to the stern, thereby making the watercraft faster and more efficient than a non-stepped hull (for example, as shown in prior art FIG. 3). In some stepped hulls, air is sucked into the transverse cavities, from where it flows into the stern portion of the boat, toward the transom. The air beneath the stern also creates lift and reduces friction between the hull and the water. Essentially, a portion of the stern rides on a cushion of air. The reduction in wetted hull surface area and the resulting reduction in friction mean that the watercraft moves more easily in the water, thereby improving efficiency of the craft. However, this also means that the stern portion of the hull becomes “slippery,” which can lead to handling, stability, and maneuverability difficulties. Further, although stepped hull designs that incorporate air channels that allow for longitudinal air flow may improve efficiency, the resulting air on the stern channels may introduce air into the water flow at the propeller and thereby reduce propeller performance and decrease propulsive efficiency, thereby offsetting gains in overall efficiency.
It is therefore desirable to provide an improved stepped hull design that increases stability and maneuverability of a watercraft, while maintaining propeller performance and propulsive efficiency.